The present invention relates generally to motor operated valve combinations and, more particularly, to a controller for such a motor operated valve for providing closure assurance upon differential valve pressure conditions.
Motor operated valve combinations or valves are commonly used in nuclear and other power plants to perform routine opening and closing of water and steam lines and, more importantly, for those valves designated as safety related, to perform such opening or closing to assure the prompt, safe shutdown of the plant under accident or other emergency or abnormal operating conditions. For those valves of this class whose safety function is to close a water, steam or other line, it is important that the valve achieve full or complete closure when called upon to do so. Most motor operated valves in use today are controlled by a torque switch which shuts off or stops the operation of the motor when a predetermined valve actuator torque limit is reached and a spring pack or other component moves in response to actuator torque in excess of the limit to trip or actuate the torque switch. Since the torque switch displacement is approximately proportional to the actuator torque above some preload torque, and since the actuator torque is approximately proportional to the stem force, the torque switch displacement set point can be and generally is used to turn off the motor at approximately the valve stem force required to fully close the valve.
However, with this method of motor control, significant stem force inaccuracies can occur for the following reasons. First, the relationship between the actuator torque developed and the torque switch displacement depends upon the rate at which the load is applied, with higher torques and higher forces being achieved at higher rates of loading. Since differential pressure conditions in the valve which may be encountered during accident, emergency or other abnormal operation conditions generally result in slower rates of loading than non-differential pressure conditions or even non-accident or emergency differential pressure conditions, the torques and forces achieved in accident or emergency conditions may be much less than those required for valve closure, even though the valve may have been demonstrated to fully close under non-accident conditions. Second, the valve stem force achieved for a given valve actuator torque depends in part on the condition of the lubrication and engaging surface conditions which exist between the valve stem and the stem nut that drives the valve stem. Lubrication degradation results in lower stem forces for a given valve actuator torque, presenting the possibility that a valve that has been shown to achieve the required forces for closure during normal operating conditions may fail to do so during subsequent accident or emergency operating conditions.
To avoid these problems, this inventor, in U.S. patent application Ser. No. 340,983, filed Jan. 3, 1989, and entitled "Thrust Actuated Control for Motor Operated Valve," now U.S. Pat. No. 4,898,363 presented an apparatus and method for controlling the motor cut-off for a motor operated valve on the basis of the total stem force. However, having the proper stem forces, though a major improvement on the prior art, still does not guarantee sufficient disc force to fully seal off the valve closure area and sufficient additional disc wedging force to prevent leakage through the valve.
Some of the stem force is used to overcome the valve packing forces and some is used to counteract stem rejection forces caused by the internal valve pressure attempting to "spit out" the stem. It would not be unusual for the packing forces to change with time due to hardening or other changes of the packing material, or to be purposefully increased by maintenance personnel attempting to stop a leak. In either event, less force is available for valve closure and for sufficient wedging force for leakage prevention. Also, the condition of the valve disc and valve seat metal sealing surfaces are important factors in determining the force required for valve closure. This force is directly proportional to the resultant disc friction factor which may be initially unknown and which may degrade with time or with repeated closures. Such degradation tends to increase the disc force required for valve closure. Finally, the differential pressure associate with an accident, emergency or other abnormal valve operation condition may have been underestimated when the plant was designed, requiring additional force for valve closure.
Thus, use of a torque switch to control motor shutoff is unacceptable because, for the reasons discussed above, in order to assure valve closure in accident or emergency conditions for all the above possible situations, multiple safety factors must be applied to the torque switch setting. The use of such multiple safety factors generally results in unacceptably high torque and force levels during normal operating conditions, levels which may be high enough to be either unachievable or damaging to the valve and/or valve actuator components.
The present invention overcomes the problems associated with the prior art and assures valve closure and sealing sufficient to prevent leakage through the valve, particularly during accident, emergency or other abnormal operating conditions by controlling motor cutoff on the basis of the valve disc achieving a desired wedging force after the valve closure area is sealed off.